The release of neurotransmitters from a nerve terminal occurs by fusion of membrane-bound vesicles with the presynaptic membrane, and dispersion of the vesicle contents into the extracellular space. We are rapidly gaining a molecular understanding of vesicle fusion, based on studies of vesicle trafficking between intracellular compartments, such as the endoplasmic reticulum and the golgi apparatus. The proteins that comprise the minimum fusion machinery have been identified, and neuronal isoforms of these proteins are essential for transmitter release. They are targets of the botulinum and tetanus toxins, neurotoxins that severely impair transmitter release at the neuromuscular junction and at inhibitory synapses in the brain, respectively. Thus synaptic vesicle fusion is a form of vesicle trafficking that is no longer unregulated - it requires a signal: calcium ions. The more calcium ions that enter the nerve terminal, the greater the amount of transmitter release. Transmitter release can be modulated by activity. Depending on experimental conditions and the synapse being studied, rapid repetitive stimulation can increase the amount of transmitter released by each stimulus (facilitation), or there can be a reduction in release with each successive impulse (depression). We do not yet understand how the basic fusion machinery is able to respond so differently to large vs. small levels of calcium influx: and to be modulated up, or down, by prior activity. Rab GTPases are a family of proteins with specific members located at each step along the pathway of intracellular membrane trafficking. Biochemical evidence suggests that rab GTPases promote the formation of the basic fusion machinery - facilitating the pairing between proteins on the vesicle and those on the target membrane. Rab3a, the rab GTPase located on synaptic vesicles, is a likely candidate for a regulator of the synaptic vesicle fusion machinery, and thus, transmitter release. In this proposal we examine transmitter release at the neuromuscular junction and in adrenal chromaffin cells expressing different levels of rab3a or a constituitively activated rab3a mutant, rab3a [Q81L], to determine if the rab3a GTPase is a modulator of transmitter release. The results of these studies will increase our ability to alter the amount of transmitter release in neurological diseases in which transmitter release is abnormal.